A Stirling engine is characterized by having an external heat source as contrasted with an internal combustion engine. The external heat source can come from combustion of fossil fuels, concentrated solar energy, heat from the decay of radioactive isotopes, hot exhaust gasses from diesel engines, or any other source of heat. Early Stirling engines used air as a working fluid, but modern ones use a gas such as Helium at pressures of 30 atmospheres or so.
There are two main methods of transmitting forces from the Stirling power piston to perform useful mechanical work on a load such as an electrical generator. In a so-called “kinematic” design, a power piston is connected to a crankshaft, as in a conventional automobile internal combustion engine, and turns a load such as a rotary electrical generator. In this case, the power piston excursion is constrained to limits established by the piston's rigid mechanical connection to the crankshaft. The second configuration is the so-called “free piston” Stirling engine (“FPSE”) wherein a mechanically unconstrained piston moves in simple harmonic motion at a frequency nominally equal to a natural mode determined by piston mass and various restoring spring rates provided by pneumatic, mechanical or other means. Typically FPSE piston displacement is controlled by appropriate dynamic balancing of input heat flux and mechanical loading to avoid excursions beyond design limits which would cause undesired impact with the cylinder ends. In one typical FPSE application, the power piston is connected by a rigid rod to a cylindrical magnetic structure (often called a “mover”) which cooperates with the fixed stator portion of a linear electrical alternator. The back and forth movement of the mover/power piston generates an AC voltage at the output of the alternator. In some applications, the FPSE configuration is preferred to its kinematic alternative, one distinct advantage being that the FPSE virtually eliminates piston-cylinder wall normal forces thereby avoiding the need to lubricate these surfaces and provides means to isolate lubricant-intolerant engine components.
A cross sectional view of a generic FPSE/linear alternator (FPSE/LA) combination 10 is illustrated in FIG. 1 with the FPSE portion 50 to the left of the figure and the alternator portion 60 to the right of the figure. A gas-tight case 12 contains a freely moving displacer 14 guided by a fixed displacer rod 16. A movable power piston 18 is connected to a permanent magnet structure 20. Various ring seals (not illustrated) may be used to form a gas tight seal between the displacer 14 and power piston 18 and internal part of the case 12. Alternatively, tight radial clearances may be used to limit leakage flows around the pistons and displacer components.
The mover employs one or more permanent magnet elements which produce a field flux in a fixed core 24 that links with turns of an armature coil 22. Motion of the mover produces a time-varying coil flux linkage and the consequent induction of a so-called “internal voltage”. The internal voltage oscillates at the frequency of the mover and with amplitude proportional to the time rate of change of coil flux linkage. When an external load is connected to the armature coil the developed internal voltage will drive current through an external load impedance and is thereby capable of delivering useful electrical power to dissipative load elements. Armature current flow through the external load in turn causes a mover reaction force which must be overcome by the power piston effort. By this means mechanical power delivered to the mover by the power piston is converted to electrical power.
Usually, the four central spaces inside the case are denominated as follows. The space between the displacer 14 and the case 12 is the expansion space 32; the space inside the displacer 14 may serve as a gas spring 34, the space between the displacer and the power piston 18 is the compression space 36; and the space between the power piston 18 and the case 12 is the bounce space 38. The case 12 may be mounted on mechanical springs (not illustrated).
Thermal energy to run the Stirling engine is supplied by a heater 40 on the outside of the case 12 opposite the displacer 14. Any source that can heat the gas in the expansion space 32 is usable. Inside the case 12, surrounding the displacer 14, is a regenerator 42. In one portion of the operating cycle, gas from the expansion space is forced through the inlet space 44 from the expansion space 32 and via the regenerator 42 through the outlet space 46 to the compression space 36. In a second portion of the cycle, gas from the compression space is returned to the expansion space via outlet space 46, regenerator 42 and inlet space 44. A cooler 48 surrounds the case 12 in the vicinity of the outlet space 46. As is well known, to achieve maximum thermodynamic efficiency, the cooler should cool the gas in the outlet space 46 as much as possible.
The operating principles of a Stirling engine are less intuitively obvious than those of a steam or internal combustion engine. U.S. Pat. No. 6,062,023, issued May 16, 2000, to Kerwin et al. for “Cantilevered Crankshaft Stirling Cycle Machine,” incorporated herein by reference, describes the four stage thermodynamic cycle of a generic Stirling engine. The Stirling engine was invented by Robert Stirling in 1816 and the basic principles are well known in the art. A brief historical review is contained in U.S. Pat. No. 5,146,750, issued Sep. 15, 1992, to Moscrip for a “Magnetoelectric Resonance Engine,” and is incorporated herein by reference.
Unlike kinematic Stirling engines, in the FPSE/LA combination 10 illustrated in FIG, 1, there is nothing to prevent displacer 14 or piston 18 from impacting parts of case 12. Designers have long sought designs that would limit piston and displacer excursion and thus prevent impacts so as to keep the engine running in a stable manner under varied operating conditions. In the case of FPSEs, heat flux input and mechanical piston power extraction (e.g., via a piston driven linear alternator and electrical load) are two controllable factors which may be employed to influence piston and displacer excursion. Unfortunately, heat flux control is not generally useful in controlling these excursions since the control can not be effected quickly enough to address out-of-bounds piston or displacer excursion limitation because, in many applications of these machines, the electrical load on the alternator can undergo very rapid changes.
Various solutions to piston and displacer excursion control under varying piston loading conditions have been utilized. One class of solutions involves maintaining control of piston and displacer excursions via self-controlling mechanisms such as porting arrangements wherein, for example, these excursions are maintained without the need for external load control. Various problems and drawbacks have been associated with these types of solutions including, for example, the fact that ports can clog with materials resulting from normal engine operation such chafing of external component surfaces over time. In addition, these prior art solutions also tend to generate losses, negatively impacting efficiency.
Another class of prior art solutions has been directed to effecting control of the electrical load placed upon the alternator being driven by the piston. In this case, a ballast or auxiliary load is used to maintain a constant load on the alternator despite changes in the external load power demand. Thus, these solutions take advantage of the fact that, for a given heat flow input to the FPSE, piston excursion can be constrained to a specific range as long as the alternator load is caused to remain constant. These solutions represent, however, inefficient techniques for controlling piston amplitudes and possible excursions resulting from load variations.
An example of a solution using constant alternator load control is U.S. Pat. No. 4,873,826, issued Oct. 17, 1989, to Dhar. Dhar discloses the control of engine operation through a connection between the alternator output and a utility grid. While this solution have many advantages, such as the ability to maintain a constant engine load, resulting operational characteristics are generally constrained by the fact that engine operation must match the operational frequency of the power grid. This constraint inhibits attainment of higher FPSE/LA power density which might otherwise be achieved by operation at a frequency higher than that of a 50 or 60 Hz power grid. Further, frequency variation over time cannot be achieved even though it is otherwise beneficial in certain circumstances such as, for example, during engine warm up when a lower pneumatic spring rate favors operation at a frequency lower than the nominal rated value, or during emergency out of range conditions where a rapid drop of enforced operating frequency can near-instantaneously reduce piston and displacer excursions. Finally, in the case of portable applications, it is often not feasible to connect to a power grid as required by Dhar.
Various prior art examples alternatively provide control without connection to a utility grid. Examples of these control systems are disclosed in U.S. Pat. No. 4,642,547, issued Feb. 10, 1987, to Redlich which teaches a control system that provides an adjustable ballast load as well as alternator armature tap connections to maintain constant engine loading as the user load varies. U.S. Pat. No. 6,050,092, issued Apr. 18, 2000, to Gentsler et al. which also controls operation using a variable load member to dynamically adjust load during operation and thereby control piston stroke. The use of alternator ballast loading to maintain constant engine load as practiced by these solutions, is inefficient. Additionally, these methods and the disclosed embodiments do not provide means to enforce variation of engine frequency which, as stated above, is desirable for a number of reasons.
As an alternative to piston stroke control by controlling the electrical load on the alternator output, various mechanical methods have been proposed using controllable valves, springs, and linkages as disclosed in U.S. Pat. No. 5,385,021, issued Jan. 31, 1995, to Beale and U.S. Pat. No. 5,502,968, issued Apr. 2, 1996, also to Beale. While these solutions claim to achieve piston stroke control, they do not exercise control of operating frequency. Additionally, the controllable rate electromagnetic spring element described would necessarily incur winding resistance, hysteretic and eddy current losses all of which compromise the overall efficiency of the power plant.